The present invention relates to a biaxially oriented polyester film to be molded and laminated on a metal sheet. More specifically, it relates to a biaxially oriented polyester film to be molded and laminated on a metal sheet, which exhibits excellent moldability when it is laminated on a metal sheet and subjected to a can making process such as drawing and from which metal cans having excellent heat resistance, retort resistance, taste and odor retention properties, impact resistance and corrosion prevention properties, such as drink cans and food cans, can be produced.
Metal cans are generally coated on interior and exterior surfaces to prevent corrosion. Recently, the development of methods for obtaining corrosion prevention properties without using an organic solvent has been promoted to simplify production process, improve sanitation and prevent pollution. One of the methods is to coat a metal can with a thermoplastic resin film. That is, studies on a method for making cans by laminating a thermoplastic resin film on a sheet of a metal such as tin, tinfree steel or aluminum and drawing the laminated metal sheet are under way. A polyolefin film or polyamide film has been tried as this thermoplastic resin film but does not satisfy all requirements such as moldability, heat resistance, impact resistance, and taste and odor retention properties.
Then, a polyester film, particularly a polyethylene terephthalate film attracts much attention as a film having well-balanced properties and there have been made some proposals based on this film (JP-A 56-10451, JP-A 64-22530, JP-A 1-192545, JP-A 1-192546 and JP-A 2-57339) (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). However, studies conducted by the present inventors have revealed that, in the case of molding which is accompanied by large deformation, moldability, retort resistance, and taste and odor retention properties become unsatisfactory.
A copolyester film has been studied as a film which is satisfactory in terms of moldability, heat resistance, impact resistance, and taste and odor retention properties. JP-A 5-339348 discloses a polyester film to be molded and laminated on a metal sheet, which comprises a copolyester having a specific melting point, glass transition temperature and terminal carboxyl group concentration. JP-A 6-39979 proposes a polyester film to be molded and laminated on a metal sheet, on which a copolyester having a specific melting point and glass transition temperature is laminated. However, studies conducted by the present inventors have revealed that when cans covered with these films are used as drink containers, a change in odor or taste is detected according to the type of a drink as disclosed by JP-A 55-23136.
JP-A 6-116376 proposes a polyester film to be molded and laminated on a metal sheet, which is made from a copolyester containing specific amounts of elemental alkali metals and elemental germanium. When this film is used, it exhibits excellent taste and odor retention properties in a system in which it is not heated while containing contents, such as a cold pack system, but does not always obtain sufficient taste and odor retention properties in a system in which it is heated while containing contents, such as a retort treatment.
JP-A 7-70340 proposes a copolyester film containing lubricant particles having an average particle diameter of 1.0 xcexcm or less and specifying the density of agglomerates of the particles in the film as a material having pinhole resistance. JP-A 8-269215 proposes a polyester film which specifies the degree of deformation of particles contained in the film as a material having pinhole resistance. When a metal sheet is deformed more than usual, sufficient can making properties cannot be obtained from the films because the particles may fall off from these films, taste and odor retention properties may deteriorate and the heat resistance of the particles may lower.
It is an object of the present invention to provide a biaxially oriented polyester film to be molded and laminated on a metal sheet, which eliminates the defects of the prior art and has improved pinhole resistance and deep drawability while retaining excellent heat resistance, impact resistance, taste and odor retention properties, and corrosion prevention properties of a copolyester film.
It is another object of the present invention to provide an industrially advantageous process for producing a polyester as a raw material for the biaxially oriented polyester film of the present invention.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, the above objects and advantages of the present invention are attained by a biaxially oriented polyester film to be molded and laminated on a metal sheet,
(A) which comprises a copolyester comprising (a) terephthalic acid and isophthalic acid as dicarboxylic acid components, terephthalic acid being contained in an amount of 82 mol % or more and isophthalic acid or a combination of isophthalic acid and other dicarboxylic acid being contained in an amount of 18 mol % or less based on the total of all the dicarboxylic acid components, and (b) ethylene glycol in an amount of 82 to 100 mol % and other diol in an amount of 0 to 18 mol % based on the total of all the diol components as diol components, having (c) a glass transition temperature of 70xc2x0 C. or higher and lower than 78xc2x0 C., (d) a melting point of 210 to 250xc2x0 C., (e) an intrinsic viscosity of 0.50 to 0.80 dl/g, and containing (f) porous particles having an average particle diameter of 0.1 to 2.5 xcexcm, a pore volume of 0.05 to 2.5 ml/g, a specific surface area of 50 to 600 m2/g and a compressive resistance of 1 to 100 MPa, and;
(B) which contains agglomerates of the porous particles having a particle diameter of 20 xcexcm or more at a density of no more than 10/mm2.
The copolyester used in the present invention consists of terephthalic acid and isophthalic acid as dicarboxylic acid components, and terephthalic aid is contained in an amount of 82 mol % or more and isophthalic acid or a combination of isophthalic acid and other dicarboxylic acid is contained in an amount of 18 mol % or less based on the total of all the dicarboxylic acid components as specified in (a). Illustrative examples of the other dicarboxylic acid other than terephthalic acid and isophthalic acid include aromatic dicarboxylic acids such as 2,6-naphthalenedicarboxylic acid and phthalic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decanedicarboxylic acid; and alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid. They may be used alone or in combination of two or more. Out of these, 2,6-naphthalenedicarboxylic acid is preferred from the viewpoints of flavor retention properties and impact resistance.
As specified in (b), the copolyester comprises ethylene glycol in an amount of 82 to 100 mol % and other diol in an amount of 0 to 18 mol % based on the total of all the diol components. Illustrative examples of the other diol include aliphatic diols such as diethylene glycol, propylene glycol, neopentyl glycol, butanediol, pentanediol and hexanediol; alicyclic diols such as cylohexane dimethanol; aromatic diols such as bisphenol A; and polyalkylene glycols such as polyethylene glycol and polypropylene glycol. They may be used alone or in combination of two or more. Out of these, triethylene glycol, cyclohexane dimethanol, neopentyl glycol and diethylene glycol are preferred from the viewpoints of flavor retention properties and moldability.
When one of these ethylene glycols is contained as the main glycol component, diethylene glycol is preferably copolymerized, more preferably copolymerized in an amount of 5 mol % or less, particularly preferably 4 mol % or less based on the total of all the glycol. When the amount of diethylene glycol is larger than 5 mol %, heat resistance may lower. This diethylene glycol component includes a diethylene glycol component by-produced when an aromatic copolyester comprising ethylene glycol as a glycol component is produced.
The copolyester of the present invention is preferably a copolyester which comprises terephthalic acid and isophthalic acid as the only dicarboxylic acid components and ethylene glycol as the only diol component.
The copolyester of the present invention has a glass transition temperature (Tg) of 70xc2x0 C. or higher and lower than 78xc2x0 C. and a melting point of 210 to 250xc2x0 C. as specified in (c) and (d). When Tg is lower than 70xc2x0 C., heat resistance deteriorates, thereby worsening taste and odor retention properties after the retort treatment of a film.
Tg of a film is measured by a method for obtaining a glass transition point with the Du Pont Instruments 910 DSC at a temperature elevation rate of 20xc2x0 C./min by placing 20 mg of a sample in a DSC measurement pan, heating and melting the sample on a heating stage at 290xc2x0 C. for 5 minutes and quenching to solidify the sample containing pan on an aluminum foil placed on ice.
When the melting point is lower than 210xc2x0 C., the heat resistance of the film deteriorates disadvantageously and when the melting point is higher than 250xc2x0 C., the crystallinity of the film becomes high, thereby impairing the moldability of the film disadvantageously. The melting point is preferably in the range of 215 to 245xc2x0 C.
The melting point of the film is measured by a method for obtaining a melting peak with the Du Pont Instruments 910 DSC at a temperature elevation rate of 20xc2x0 C./min. The amount of the sample is 20 mg.
The copolyester of the present invention has an intrinsic viscosity (o-chlorophenol, 35xc2x0 C.) of 0.50 to 0.80 dl/g as specified in (e). When the intrinsic viscosity is lower than 0.50, the impact resistance of the film becomes insufficient disadvantageously and when the intrinsic viscosity is higher than 0.80, the intrinsic viscosity of a raw material polymer must be increased excessively which is economically disadvantageous. It is preferably 0.55 to 0.75, more preferably 0.60 to 0.70.
Preferably, the copolyester of the present invention has a terminal carboxyl group concentration of 40 eq./106 g or less and an aldehyde content of 15 ppm or less.
Preferably, the copolyester of the present invention contains alkali metal compounds in a total amount of 5 ppm or less in terms of elemental alkali metals from the viewpoint of taste and odor retention properties. The total amount of elemental alkali metals is the total amount (ppm) of elemental Li, Na and K determined by atomic absorption spectrophotometry.
A catalyst used in the polymerization reaction of the copolyester is not limited to a particular kind but preferably an antimony compound (Sb compound), titanium compound (Ti compound) or germanium compound (Ge compound). Out of these, a germanium compound is particularly preferred from the viewpoint of the taste and odor retention properties of the obtained film.
Preferred examples of the antimony compound include antimony trioxide, antimony acetate and the like. Preferred examples of the titanium compound include titanium tetrabutoxide, titanium acetate and the like. Preferred examples of the germanium compound include amorphous germanium oxide, fine crystalline germanium oxide, a solution prepared by dissolving germanium oxide in a glycol in the presence of an alkali metal, alkali earth metal or a compound thereof, a solution prepared by dissolving germanium oxide in water, and the like.
The process for producing the copolyester of the present invention is not particularly limited. The copolyester of the present invention is produced in the presence of a metal compound as a catalyst and a phosphorus compound as a stabilizer and satisfies the following expressions (1) and (2):
20xe2x89xa6M+Pxe2x89xa655xe2x80x83xe2x80x83(1)
1xe2x89xa6M/Pxe2x89xa65xe2x80x83xe2x80x83(2)
wherein M is the concentration (mmol %) of a metal element contained in the copolyester and P is the concentration (mmol %) of phosphorus element contained in the copolyester.
When (M+P) is smaller than 20 mmol %, the copolyester productivity of an electrostatic casting method lowers. When (M+P) is larger than 55 mmol %, the amount of an ether glycol by-produced may increase, thereby reducing heat resistance. When M/P is smaller than 1 or larger than 5, the ratio of the metal element of the catalyst and phosphorus element may get out of balance and an excess of phosphorus element or the metal element of the catalyst may be contained in the polymer, thereby reducing thermal stability.
Further, the metal element (M) of the catalyst is preferably in the range of 10 to 35 mmol %. When M is smaller than 10 mmol %, it is difficult to obtain a copolyester having a sufficient degree of polymerization and characteristic properties such as impact resistance may lower. When M is larger than 35 mmol %, the thermal stability of the obtained copolyester may lower.
The above copolyester of the present invention further contains porous particles having an average particle diameter of 0.1 to 2.5 xcexcm, a pore volume of 0.05 to 2.5 ml/g, a specific surface area of 50 to 600 m2/g and a compressive resistance of 1 to 100 MPa as specified in (f). The average particle diameter is preferably 0.1 to 1.5 xcexcm, more preferably 0.3 to 1.0 xcexcm.
When the average particle diameter is larger than 2.5 xcexcm, pinholes are easily formed at the time of molding disadvantageously.
The average particle diameter of the porous particles is a value at an integral 50% point in an equivalent sphere diameter distribution obtained by a centrifugal sedimentation particle size distribution measuring instrument.
The pore volume of each of the porous particles is 0.05 to 2.5 ml/g as described above, preferably 0.1 to 2.0 ml/g, more preferably 0.5 to 1.8 ml/g. When the pore volume of the porous particle is smaller than 0.05 ml/g, the affinity of the porous particle for the film lowers, thereby causing the breakage of the film at the time of molding. When it is larger than 2.5 ml/g, the porous particles are reduced in size at the time of molding and some of the particles are easily contained in a drink, thereby reducing taste and odor retention properties disadvantageously.
The pore volume of the porous particle is measured by a mercury-helium method.
The specific surface area of the porous particle is 50 to 600 m2/g as described above, preferably 150 to 450 m2/g.
The compressive resistance of the porous particle used in the present invention is 1 to 100 MPa, more preferably 5 to 50 MPa. When the compressive resistance is larger than 100 MPa, the porous particle itself chips the film at the time of molding, thereby reducing impact resistance and corrosion preventing properties.
The compressive resistance is defined as follows. Load is applied to the porous particle by a micro compression tester while the particle is observed through a microscope to obtain a load at break, this operation is made on at least 100 porous particles, and the mean value of measurement data on the 100 particles is taken as compressive resistance.
The content of the porous particles used in the present invention is preferably 0.05 to 5.0 wt %, more preferably 0.08 to 3.0 wt %, much more preferably 0.1 to 1.0 wt %. When the content is smaller than 0.05 wt %, film winding properties become unsatisfactory, thereby reducing productivity and when the content is larger than 5.0 wt %, pinholes are easily formed in the film at the time of molding disadvantageously.
The porous particles used in the present invention are inorganic particles such as colloidal silica, porous silica, titanium oxide, calcium carbonate, calcium phosphate, barium sulfate, alumina, zirconia, kaolin or composite oxide particles; or organic particles such as crosslinked polystyrene, acrylic crosslinked particles, methacrylic crosslinked particles or slicone particles. The porous particles are not limited to the above externally added particles and may be internally precipitated particles obtained by precipitating part or all of the catalyst used in the production of a copolyester in a reaction step. Externally added particles and internally precipitated particles may be used in combination. Out of these, porous silica having a large specific surface area is particularly preferred from the viewpoint of moldability.
The porous particles have hydroxyl groups on the surface in an amount of preferably 300 KOHmg/g or less, more preferably 200 KOHmg/g or less in terms of hydroxyl value.
The copolyester of the present invention may further contain inert globular particles which have a particle diameter ratio (long diameter/short diameter) of 1.0 to 1.2 and an average particle diameter of 2.5 xcexcm or less and do not substantially agglomerate, in addition to the above porous particles. Illustrative examples of the inert globular particle lubricant include inorganic lubricants such as silica, alumina, titanium oxide, calcium carbonate and barium sulfate; and organic lubricants such as silicone resin particles and crosslinked polystyrene particles. Out of these, spherical silica, spherical silicone resin particles and globular crosslinked polystyrene are particularly preferred.
Preferably, these globular lubricants have an average particle diameter of 0.05 to 0.8 xcexcm which is smaller than the average particle diameter of the porous particles. The content thereof is preferably 0.01 to 1 wt %.
The copolyester of the present invention can be advantageously produced by esterifying terephthalic acid and isophthalic acid and optionally other dicarboxylic acid as dicarboxylic acid components and ethylene glycol and optionally other diol as diol components; and adding porous particles having an average particle diameter of 0.1 to 2.5 xcexcm, a pore volume of 0.05 to 2.5 ml/g, a specific surface area of 50 to 600 m2/g and a compressive resistance of 1 to 100 MPa to a polycondensation reaction system after the terminal carboxyl group concentration of the obtained polycondensate becomes 100 eq./106 g or less.
By adding the porous particles after the terminal carboxyl group concentration of the copolyester (precursor) becomes 100 eq./106 g or less, the agglomeration of the porous particles into coarse particles can be suppressed and the porous particles can be well dispersed.
The terminal carboxyl group concentration can be obtained by an A. Conix method (Markromol. Chem. 26, 226 (1958)).
The copolyester obtained by the above method (melt polymerization) may be converted into a polymer having a higher degree of polymerization by a polymerization method (solid-phase polymerization) in a solid phase as required.
The copolyester may contain optional additives such as an antioxidant, thermal stabilizer, viscosity modifier, plasticizer, color modifier, nucleating agent and ultraviolet light absorber.
The polyester film of the present invention is a biaxially oriented film obtained by biaxial orientation and optionally heat setting.
The biaxially oriented polyester film of the present invention contains coarse particles having a particle diameter of 20 xcexcm or more at a density of no more than 10/mm2. The coarse particles are specified as the agglomerates of the above porous particles. When the density of the coarse particles having a particle diameter of 20 xcexcm or more is higher than 10/mm2, pinholes are formed at the time of molding.
The refractive index in a thickness direction of the polyester film of the present invention is preferably 1.500 to 1.540, more preferably 1.505 to 1.530. When the refractive index is too low, moldability becomes unsatisfactory and when the refractive index is too high, the film becomes almost amorphous, whereby heat resistance may lower.
The center line average roughness (Ra) on the film plane of the polyester film of the present invention is preferably 30 nm or less, more preferably 25 nm or less, particularly preferably 20 nm or less.
The polyester film of the present invention has a thickness of preferably 6 to 75 xcexcm, more preferably 8 to 75 xcexcm, particularly preferably 10 to 50 xcexcm. When the thickness is smaller than 6 xcexcm, the film is easily broken at the time of molding and when the thickness is larger than 75 xcexcm, the quality of the film becomes excessively high, which is economically disadvantageous.
The metal sheet to be laminated with the polyester film of the present invention, particularly a metal sheet for can making, is advantageously a sheet of tin, tin-free steel, aluminum or the like. The polyester film can be laminated on the metal sheet by the following methods.
(1) The metal sheet is heated at a temperature higher than the melting point of the film, laminated with the film and cooled to make a surface layer portion (thin layer portion) of the film bonding to the metal sheet amorphous to contact with the metal sheet.
(2) A primer is coated on the film to form an adhesive layer and the film is laminated on the metal sheet in such a manner that the adhesive layer is in contact with the metal sheet. Known resin adhesives such as epoxy adhesives, epoxy-ester adhesives and alkyd adhesives may be used to form the adhesive layer.
A laminate produced by laminating the polyester film of the present invention with a metal sheet as described above can be advantageously used for the production of a metal can having the polyester film on the interior side by deep drawing.